I do a lot of reverse image searching. I have never found a reverse image search that reaches into videos, on YouTube or elsewhere. I would be super-happy to learn of one that I currently don't know about.
I've had extremely minor luck (which is to say, a few searches have worked, out of dozens) by grabbing a still frame from a video and doing reverse image searches on that. But I can't say I ever got Youtube results.
Here is my "secret weapon" for reverse image searches: a single one-page internet site that lists every reverse image search tool in existence (and a bunch of other handy image-handling and image-investigation tools):
ImgOps
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